Field of the Invention
The invention relates to a sheet-conveying device for machines used in the printing technology field.
It has become known heretofore to hold thin workpieces such as sheets firmly and transport them with the aid of electrostatic devices. U.S. Pat. No. 4,244,465 discloses a device of that general type, whereby sheets are transported on a conveyor belt, wherein two groups of strip-shaped, equally spaced electrodes are integrated. The electrodes are surrounded by an insulating material and are connected to a high-voltage source, so that an electrostatic field is produced across the surface of the conveyor belt. A disadvantage of such a construction is that the electrodes revolve with the conveyor belt, which results in an increase in wear and tear of the electrodes and the conveyor belt. Furthermore, the structure of the electrodes rises above the surface of the conveyor belt, so that the contact surface is not completely even, which can be disadvantageous when transporting and processing thin sheets. The holding forces acting upon the sheets are reduced due to surface discharges; it may thereby become necessary to change the high-voltage polarity. The inhomogenous field created by the electrodes cannot be compensated for completely by the sheets, so that an increased deposit of dust is produced upon the conveyor belt. Due to a parasitic corona which can be formed when the sheets are removed from the conveyor belt, surface charges accumulate in the insulating layer covering the electrodes. The surface of the conveyor belt may thereby become passivated, and the holding forces acting upon the sheet may accordingly be lost.
U.S. Pat. No. 4,526,357 describes a sheet separating or singling device which is based upon the same principle as that mentioned hereinabove.
The published European Patent Document EP 0 297 227 A2 shows an electrostatic holding device having electrodes pairwise embedded in a basic material and being connected to voltage sources which alternatingly change their polarity.
The published German Patent Document DE 4012 210 A1 discloses a sheet-conveying device with an endless belt wherein no electrodes are provided in the material of the belt. With the aid of an electrode which extends across the width of the belt and is in contact with an a.c.-voltage source, a charge-density pattern is formed on the surface of the belt by touching. The resulting non-uniform electrical field creates reflecting charges in the material of the sheets; thereby a holding power is generated to keep the sheets on the surface of the belt.
In order to attain uniform holding forces, the frequency of the a.c. voltage should be in phase with the revolving speed of the belt, which calls for expenditures involving control technology. Because the in-phase state cannot be realized completely, positively charged fields, for example, are charged negatively during the succeeding revolution of the belt. A corona effect produced by this change in charge causes ozone and nitrogen oxide to escape into the environment. The consumption of energy is increased. Particularly when the spacing between the positively and the negatively charged fields on the surface of the belt is rather slight, more changes in charge occur both when the belt runs into the effective range as well as when it runs out of the effective range of the charge electrode.
The use of a.c. voltage increases the tendency towards sliding discharges along the insulating surface of the belt. Due to the finite ohmic resistance on the surface of the belt, distances or spacings between the charges of more than 1 mm are optimal. It is thereby possible to deposit the sheets on the belt in such a manner that the edges of the sheets are at a given distance from a charge extremum. The maximal holding force cannot thereby act upon the sheet edges, which would be desirable for many uses.
In the construction according to the published German Patent Document DE 4012 210 A1, a blade-shaped electrode or a charge roller is provided having a large spatial range. When using high a.c. voltages, capacitive interferences with electronic control circuits may occur, which can only be reduced by providing additional shields, filters, and the like.
If the charging roller is to be used simultaneously as a tensioning roller for a belt having an inner conductivity, then a high capacity exists between the charging roller and the belt due to the looping angle. When a.c. voltage is applied, a high blind power and a high energy demand, respectively, result.
In U.S. Pat. No. 3,726,520, there is described a separating or singling device for sheets at the top of a sheet pile, whereby a cyclical reciprocally displaced foil or an endless belt being charged to a defined potential by a corona-charge source is used as a conveyor element. As the transported sheet approaches, it is attracted to the charged conveyor element and held there until a discharge of the conveyor element takes place. The sheets are also charged by static induction, however, the danger then exists that the sheets retain residual charges after they leave the conveyor element, which may be a hindrance in the further transport or further processing of the sheets. Particularly in electrostatic printing devices, residual charges on the printing material are a cause of printing errors or misprints.